The present invention relates to thermionic cathodes or so-called hot cathodes, and a process for preparing them. More particularly, it relates to a thermionic cathode formed using a novel emission material having a comparatively low operating temperature. The process of the present invention manufactures these cathodes without hot melting or machining.
Over the years thermionic cathodes have been made in various forms to achieve high current densities, low evaporation and long life. In one of its simplest forms, the cathode comprises an emission material sprayed or painted on the surface of a support member such as tungsten, nickel or molybdenum. Another generic group of cathodes is the dispenser cathodes in which the emission material is contained in and uniformly distributed throughout a porous body of tungsten. These cathodes are able to slowly dispense emission material through pores to the emission surface such that as the surface is depleted of emission material, it is replenished with material supplied from within the body of the cathode. Dispenser cathodes are typically manufactured by impregnating the pre-formed porous body of a refractory metal with a hot melt of the emission material.
The emission material most frequently used in the art is barium oxide. Barium oxide, however, is extremely hygroscopic and readily converts to barium hydroxide, which is more stable (less readily decomposed) and, therefore, less induced to emitting an electron. Barium oxide cathodes, therefore, must be handled and stored under a carefully maintained water free atmosphere. An alternative to barium oxide cathodes are barium carbonate cathodes which are not so reactive with moisture and will convert to barium oxide at elevated temperatures and release the desired electron emission.
Cronin has disclosed several examples of thermionic cathodes in U.S. Pat. Nos. 3,656,020; 3,760,218; and 3,922,428 in which in addition to barium oxide, the emission material includes a calcium oxide and lithium oxide (U.S. Pat. No. 3,656,020), one or more of cobalt oxide, manganese oxide and molybdenum oxide (U.S. Pat. No. 3,760,218), or samarium oxide (U.S. Pat. No. 3,922,428).
For conventional thermionic cathodes, the operating temperature is above 800.degree. C. and in some cases as high as 1,000.degree. to 1,150.degree. C. Typically, the average current density of an oxide-type cathode is limited to 0.25 A/cm.sup.2 at 800.degree. C. Dispenser cathodes, on the other hand, which are fabricated by infiltrating a porous support with emission material, generally possess outputs ranging from 1 to 6 A/cm.sup.2 average at temperatures of 1000.degree. C. to 1150.degree. C. High operating temperatures as well as complex machinery procedures have complicated the use and manufacture of thermionic cathodes and made them much more expensive. In particular, the cathode body or support, must be able to withstand the high temperatures. As a general rule, the cathode should not be operated at temperatures greater than half the melting point of the cathode body because the metal diffusion rates usually encountered at higher temperatures plug the pores of a dispenser-type cathode and terminate operation. As a result many conventional cathodes employ expensive and difficult to fabricate heat-resistant refractory metals to support the emission materials.
Thus, there is a need for thermionic cathodes which provide outputs comparable to conventional cathodes but at lower temperatures.